Modem broadband communications systems are generally implemented as providing either asymmetric or symmetric service. In asymmetric service data is transmitted in one direction, typically downstream towards the customer premises, at a greater data rate than data is transmitted in the other direction, typically upstream towards the local exchange. Asymmetric services are used in environments such as the Internet, where the ratio between upstream and downstream throughput depends on the characteristics of the applications requiring data transmission. For example, downloading a picture or a movie from an ISP typically requires a large downstream throughput, while a much lower upstream throughput is used for sending data requests and acknowledgements. The typical ratio between upstream and downstream for such applications is about 1:10, ADSL modems and certain configurations of VDSL modems are designed to support asymmetric services, having a much higher downstream bandwidth than upstream bandwidths.
In symmetric service data is transmitted between two points at the same data rate. Symmetric service include T1 and E1 and their aggregates in the form of DS1 and DS3. Modems which are designed to support symmetric services include HDSL modems and some configurations of VDSL modems. Optical devices also are designed for symmetric transmission.
During the wake up process of a modem providing asymmetric service the modem typically determines both the maximum achievable upstream throughput and the maximum achievable downstream throughput give predefined SNR tolerances and a target asymmetric service ratio. The modem then sets the actual upstream throughput equal to the maximum achievable upstream throughput, and sets the actual downstream throughput equal to the maximum achievable downstream throughput. Thus, upstream throughput and downstream throughput are set independently to achieve maximum throughput in both directions.
In contrast to asymmetric service, during the wake up process of a modem providing symmetric service the modem typically determines both the maximum achievable upstream throughput and the maximum achievable downstream throughput, selects the lesser of these throughputs as the working throughput, and sets the actual upstream and downstream throughputs equal to the working throughput. As a result of this wake up process, bandwidth is often wasted in one of the two directions.
In an environment where one modem pool communicates with another modem pool, aggregate symmetric service may be provided by using only symmetric modems in each modem pool. Should a modem pool include asymmetric modems as well, aggregate symmetric service may nonetheless be provided by pairing two asymmetric modems in the one of the modem pools with a corresponding two asymmetric modems in the other modem pool and creating a single, full-duplex symmetrical circuit from the high-bandwidth channels of both asymmetric modems, such as is described in U.S. Pat. No. 6,021,120 to Beyda et al. Such a solution is disadvantageous, however, as each modem pool would require an even number of asymmetric modems. Should either modem pool have an odd number of asymmetric modems, the odd-numbered modems could not be used for aggregate symmetric service given the prior art. Furthermore, the low-bandwidth asymmetric channels would not be fully available for data transmission.